Despite his severe racism and inextricable personal commitments to slavery, Thomas Jefferson made profoundly significant contributions to the rise of anti-slavery constitutionalism. This Article examines the narrowly defeated anti-slavery plank in the Territorial Governance Act drafted by Jefferson and ratified by Congress in 1784. The provision would have prohibited slavery in all new states carved out of the western territories ceded to the national government established under the Articles of Confederation. The Act set out the principle that new states would be admitted to the Union on equal terms with existing members, and provided the blueprint for the Republican Guarantee Clause and prohibitions against titles of nobility in the United States Constitution of 1788. The defeated anti-slavery plank inspired the anti-slavery proviso successfully passed into law with the Northwest Ordinance of 1787. Unlike that Ordinance's famous anti-slavery clause, Jefferson's defeated provision would have applied south as well as north of the Ohio River.